Warning lights are often used within buildings in conjunction with audio warning alarms so that the hearing impaired can be alerted to emergency conditions such as a fire. Typically, the warning light includes a flashing bulb positioned horizontally within a reflector. A Fresnel lens may be used to spread the reflected light. The bulb may be powered by a battery source to ensure that the warning light will have power in the event power to the building is disrupted.
Warning lights approved by Underwriters Laboratories (UL) are subject to light intensity requirements. Underwriters Laboratories measures the light intensity of a warning light being tested along intersecting horizontal and vertical planes which extend outwardly from the base of the warning light. The standards are found in UL 1971 (and UL 1638), "Standard for Safety Signalling Devices for the Hearing Impaired." These standards provide that the light intensity emitted by a warning light at a given angle on each of the horizontal and vertical planes must be at a certain level at a distance of 10 feet. The UL standards provide for a maximum light intensity output of 110 candela-sec at 10 ft. for ceiling and wall mounted warning lights along an axis defined by the intersection of the horizontal and vertical planes. The UL Standards provide for lesser light intensities at angles away from the axis.